The present invention relates to an image forming method capable of controlling the recording density of, for example, an electrophotographic apparatus and, more particularly, to an image forming method which insures adequate image formation at all times.
In an electrophotographic image forming apparatus, a latent image is electrostatically formed on an image carrier by a predetermined method and then developed by a toner fed from a developing unit. Usually, the toner is charged to polarity opposite to that of the latent image so as to be electrostatically deposited on the latent image. To charge the toner to the above-mentioned polarity, use is often made of a two component type developer, i.e., a mixture of toner and carrier. As the toner and carrier of this type of developer are mixed and agitated together, the toner is charged by friction. Development using the two component developer can charge the toner to a sufficient degree. However, control for maintaining the toner concentration of the developer, i.e., the image density constant is the prerequisite since only the toner is consumed by repetitive development. To meet this requirement, it has been customary to measure the toner concentration of the developer and control the supplement of toner on the basis of the result of measurement.
To measure the toner concentration of the developer, an indirect and a direct method are available. The indirect method forms an electrostatic latent image of a particular pattern or reference pattern on a photoconductive element, develops it, and then photoelectrically measures the density of the developed image by an optical sensor. The direct method measures the weight or permeability of the developer by a toner sensor.
The conventional image forming method starts supplementing the toner only after the toner concentration has been lowered. This brings about a problem that when documents of the kind consuming a great amount of toner are continuously reproduced, a supplement sharply changes the toner concentration, making it difficult to maintain the toner concentration stable.
Another problem is that the conventional method does not take account of the time lag between a toner supplement and an increase in toner concentration. Hence, the toner concentration varies over a noticeable range, i.e., the control accuracy is not satisfactory.
Still another problem is that the amount of toner to be consumed between consecutive patterns for control is noticeably effected by the pixel density of documents, varying environment and so forth, preventing an adequate amount of toner matching the toner consumption from being supplemented. At this instant, a change in the pixel density of documents between consecutive patterns for control, i.e., a change in the amount of toner consumption disturbs a feedback system associated with the optical sensor. To enhance accuracy of toner concentration, the number of times that the pattern for control is formed and, therefore, the amount of feedback data may be increased. This, however, aggravates the consumption of toner as well as the load acting on a cleaning unit.
Japanese Patent Laid-Open Publication No. 33704/1989 discloses an image forming method using first sensing means for determining an amount of toner consumed for reproduction by counting image form signals, and second means for determining an amount of toner scattered around on the basis of the operation time of the developing roller. Based on such amounts of toner consumption, this method supplies a toner to maintain the concentration constant. However, the relation between image form signals and amounts of toner consumption is not constant since it is influenced by changes in the charging ability of the carrier ascribable to the deterioration of the developer due to aging. It follows that the ability of the developing unit changes and makes it difficult to insure an ideal image quality or toner concentration in matching relation to the varying conditions.
Generally, regarding the two component developer for electrophotography, the charging ability of the carrier decreases due to the degradation of the developer ascribed to aging. In addition, the degree of charge accumulation and, therefore, Q/M increases in a low temperature, low humidity environment. By contrast, in a high temperature, high humidity environment, Q/M decreases since the degree of charge leak increases. It has ben customary to determine a control value by considering the influence of only one or two factors separately despite that many factors effect Q/M in combination, i.e., despite that an optimum control value has to be determined in consideration of multiple information to which the target is susceptible.
In addition, with the conventional method, it is impossible to form many patterns for control when it comes to a high speed machine which is severely restricted in respect of time. This, coupled with the fact that the control processing has to be executed at high speed, obstructs accurate control.